It is well known that the turbine blade tip clearance, being the radial distance between the stationary engine casing and the tips of the rotating aerofoil turbine blades, has a significant effect on the efficiency of the turbine.
During operation of the gas turbine engine, the turbine blades are subjected to radially outwardly directed centrifugal forces resulting from the rotation of the turbine assembly.
These centrifugal forces, in combination with the high operating temperature and pressure of the gas passing through the engine result in significant changes in turbine blade tip clearance. The variation in engine operating conditions resulting from the different demands of a flight cycle can result in frequent variation in the turbine blade tip clearance.
It is well known that the efficiency of a gas turbine engine is inversely proportional to the turbine blade tip clearance. Large turbine blade tip clearances result in leakage of high pressure gas past the blade tip, which in turn causes a loss in engine efficiency. If, on the other hand, the turbine blade tip clearance is too small, then the turbine blades may rub against the internal surface of the casing causing damage to casing and/or turbine blades.
A conventional case cooling system for a gas turbine engine employs a cooling air flow directed through a manifold arrangement, with the air flow being regulated by one or more valves. Typically, such a system is configured using predicted nominal flow characteristics for the manifold(s) and valves(s).
There are two main sources of variability in such conventional case cooling systems. The first is the valve, which is the dominant source of flow variability at low cooling air flow rates. The second is the total hole area of the manifold impingement jets, which are the dominant source of flow variability at high cooling air flow rates.
Such a conventional case cooling system typically either equalises the tip clearances at the sea level high power and altitude high power conditions, or alternatively maintains the tip clearances at the sea level high power and altitude high power conditions in a known disposition relative to one another.
Consequently, any difference in the flow characteristics of the valve(s) and/or the manifold(s) will result in this tip clearance relationship deviating from that of the design and thus being detrimental to the performance of the case cooling system. For example, a low-flowing manifold in combination with a high-flowing valve can be detrimental to the altitude high power condition. Conversely, a high-flowing manifold in combination with a low-flowing valve can be detrimental to the sea level high power condition.
This variation in cooling flow characteristics and the consequent variation in tip clearance control, from engine to engine can be detrimental to the on-wing control of multiple engines.
There is therefore a requirement to calibrate the case cooling system so as to maintain the rotor blade tip clearance within a predetermined range, across the range of operation of the gas turbine engine, in order to reduce the variation in rotor blade tip clearance from engine to engine.